Statistical Characteristics of the Long-Term Variations in Major Sudden Stratospheric Warming Events 

 We investigate the statistical characteristics and the long-term variations of major sudden stratospheric warming (SSW) events in the Northern Hemisphere. We find that the strength and duration of major SSW events have increased from 1958 to 2019 and that this is due to the strengthening of the winter planetary wave activity. We find that the frequency of displacement and split SSW events differs between early, middle, and late winter. Early and middle winter are dominated by displacement and split SSW events, respectively, but the frequency of the two types of events is almost equal in late winter. This is due to the differences in the relative strength of wavenumber-1 and wavenumber-2 planetary wave activity in the three winter periods. As a result of the increase in upward planetary wave activity and the decrease in westerly winds around the polar vortex in middle winter, a shift in the timing of SSW events toward middle winter is detected. In addition, we revealed the influence of the downward propagation of different types of SSW events on the surface temperature anomaly. There were surface cold centers in Russia and northern China after the middle split SSW events; by contrast, there were more cold events in North America after the middle split SSW events.